It is well known how to fabricate thermal IR emitters. Such devices typically consist of a resistive micro-heater embedded within a thin membrane and supported on a substrate. When current is passed through the heater, it heats up to a high temperature (which can be as much as 700° C. or even higher), and at this high temperature the device emits IR radiation.
A large number of designs with IR emitters have been reported. For example, Parameswaran et al. “Micro-machined thermal emitter from a commercial CMOS process,” IEEE EDL 1991 reports a polysilicon heater for IR applications made in CMOS technology, with a front side etch to suspend the heater and hence reduce power consumption. Similarly, Bauer et al. “Design and fabrication of a thermal infrared emitter” Sens & Act A 1996, also describes an IR source using a suspended polysilicon heater. U.S. Pat. No. 5,285,131 by Muller et al., US20080272389 by Rogne et al., and San et al. “A silicon micromachined infrared emitter based on SOI wafer” (Proc of SPIE 2007) also describe similar devices using a polysilicon heater.
Yuasa et al. “Single Crystal Silicon Micromachined Pulsed Infrared Light Source” Transducers 1997, describe an infrared emitter using a suspended boron doped single crystal silicon heater. In EP2056337 by Watanabe et al. describes a suspended silicon filament as an IR source. The device is vacuum sealed by bonding a second substrate.
Many designs based on a platinum heater have also been described. For example, Hildenbrand et al. “Micromachined Mid-Infrared Emitter for Fast Transient Temperature Operation for Optical Gas Sensing Systems”, IEEE Sensor 2008 Conference, reports on a platinum heater on suspended membrane for IR applications.
Similarly Ji et al. “A MEMS IR Thermal Source For NDIR Gas Sensors” (IEEE 2006) and Barritault et al. “Mid-IR source based on a free-standing microhotplate for autonomous CO2 sensing in indoor applications” (Sensors & Actuators A 2011), Weber et al. “Improved design for fast modulating IR sources”, Spannhake et al. “High-temperature MEMS Heater Platforms: Long-term Performance of Metal and Semiconductor Heater Materials” (Sensors 2006) also describe platinum based as well as other emitters.
Some other IR emitter designs are given by U.S. Pat. No. 6,297,511 by Syllaios et al., U.S. Pat. Nos. 5,500,569 5,644,676, 5827438 by Blomberg et al., and WO2002080620 by Pollien et al.
One limitation of many of these devices is that their emissivity is not optimal and specifically for the emitter, there is the possibility of electro-migration of the metal layer. There is also little control over the emission at specific wavelengths. For this purpose, the devices are often coated with different materials to improve the emissivity. Some materials used are metal blacks, carbon, carbon nanotubes and other thin film interference structures.
There have been several reports in literature that suggest that the emissivity of devices can be varied at particular wavelengths by using plasmonic structures, which are periodic structures created on a surface. For example these are described in Shklover et al., “High-Temperature Photonic Structures, Thermal Barrier Coatings, Infrared Sources and Other Applications,” Journal of Computational and Theoretical Nanoscience, Vol. 5, 2008, pp. 862-893.
There are also several reports of IR emitters with plasmonic structures. For example, Tsai et al., “Two Color Squared-Lattice Plasmonic Thermal Emitter,” Proceedings of Sixth IEEE Conference on Nanotechnology, Vol 2, pp 746-748, describes a silver/silicon dioxide/silver sandwich structure on a silicon substrate, where the top silver and/or silicon dioxide layer have a periodic pattern. The emission spectrum of the device shows peaks near 4 μm and 6 μm. Very similar devices are also described in Jiang et al., “Enhancement of thermal radiation in plasmonic thermal emitter by surface plasmon resonance,” Proceeding of IEEE conference on Nanotechnology 2008, pp. 104-107, and in Fu et al., “A thermal emitter with selective wavelength: Based on the coupling between photonic crystals and surface plasmon polaritions,” Journal of Applied Physics 105, 033505 (2009). Huang et al., “Triple peaks plasmonic thermal emitter with selectable wavelength using periodic block pattern as top layer,” Proceedings of IEEE International Conference on Nanotechnology 2011 pp. 1267-1270, also describe a device based on silicon dioxide and silver layers, but using block shapes in different patterns at the top surface.
While all these designs are made to optimise the emission spectrum of the surface, these devices do not have an efficient mechanism for heating up. Either the heater is based on a metal layer at the back surface or need to be coupled to an external heater, which can result in very high power consumption. Unlike conventional miniaturised IR emitters, none of these devices are based on a membrane to isolate the heat and reduce power consumption.
There are a number of reports on MEMS based IR emitters with plasmonic structures. Ji et al. “Narrow-band Midinfrared Thermal Emitter Based on Photonic Crystal for NDIR Gas Sensor,” Proceedings of IEEE ICSICT 2010, pp. 1459-1461 describes a platinum heater on top of a silicon nitride/silicon dioxide/silicon composite membrane, where all these layers are patterned with an array of holes in a square pattern. Li et al. “MEMS-based plasmon infrared emitter with hexagonal hole arrays perforated in the Al—SiO2 structure,” Journal of Micromechanics and Micro-engineering 21 (2011) 105023, describe an aluminium heater on an silicon dioxide/silicon membrane, and all these layers have circular holes in them in a hexagonal pattern. While these designs will have lower power consumption due to the use of a membrane for thermal isolation, making holes through most of the membrane layers requires extra process steps, and can also structurally weaken the membrane as many of the layers, including the silicon dioxide layers, are perforated.
Puscasu et al. “Plasmonic Photonic Crystal MEMS Emitter for Combat ID,” Proc of SPIE Vol 8031, 80312Y, describes a plasmonic structure coupled with a MEMS platform. The plasmonic structure consists of circular holes in a hexagonal pattern, while the platform is a heater suspended on a micro-bridge type membrane. Similarly, Sawada et al. “Surface Plasmon Polarities Based Wavelength Selective IR Emitter Combined with Microheater,” proceedings of IEEE conference on Optical MEMS and Nanophotonics 2013, pp. 45-46, also describes an IR emitter which is suspended. Such suspended structures are less stable than full membrane structures.
Zoysa et al. “Conversion of broadband to narrowband thermal emission through energy recycling,” Nature Photonics 2012, 20.12.146, describe an IR emitter based on a gallium arsenide (GaAs) substrate and a membrane consisting of GaAs/Al—GaAs heterostructures, with the membrane layers patterned into holes in a hexagonal pattern. In the report the heterostructure is not formed within the membrane region.
It is also known how to fabricate thermal IR detectors consisting of a thin membrane layer (made of electrically insulating layers) that is formed by etching part of the substrate. Incident IR radiation increases the membrane temperature, which can be measured by either a thermopile, a resistor, or a diode.
For example, Schneeberger et al. “Optimized CMOS Infrared Detector Microsystems,” Proc IEEE Tencon. 1995, reports fabrication of CMOS IR detectors based on thermopiles. The thermopile consists of several thermocouples connected in series. KOH is used to etch the membrane and improve the thermal isolation. Each thermocouple consists of 2 strips of different materials, connected electrically and forming a thermal junction at one end (termed hot junction) while the other ends of the material are electrically connected to other thermocouples in series forming a thermal cold junction. The hot junctions of the thermocouples are on the membrane, while the cold junction is outside the membrane. Three different designs of the thermocouples are given in the paper with different material compositions: aluminium and p-doped polysilicon, aluminium and n-doped polysilicon, or p-doped polysilicon and n-doped polysilicon. Incident IR radiation causes a small increase in temperature of the membrane. The Seebeck effect causes a slight voltage difference across each thermocouple, resulting in a much large increase in voltage difference across the thermopile which is the sum of the voltages across each thermocouple.
Allison et al. “A bulk micromachined silicon thermopile with high sensitivity,” Sensors and Actuators A 104 2003 32-39, describes a thermopile based on single crystal silicon p-doped and n-doped materials. Lahiji et al., “A Batch-fabricated Silicon Thermopile Infrared Detector,” IEEE Transactions on Electron Devices” 1992, describe two thermopile IR detectors, one based on bismuth-antimony thermocouples, and the other based on polysilicon and gold thermocouples.
U.S. Pat. No. 7,785,002 by Dewes et al. describes an IR detector with a thermopile based on p-and n-doped polysilicon. Langgenhager et al. “Thermoelectric Infrared Sensors by CMOS Technology,” IEEE EDL 1992, describes IR detectors consisting of thermopiles on a suspended structure consisting of aluminium and polysilicon.
Several other thermopile devices are described by Graf et al. “Review of micromachined thermopiles for infrared detection,” Meas. Sci. Technol. 2007.
Similarly, thermodiode based IR detectors can also be made. Kim et al. “A new uncooled thermal infrared detector using silicon diode” Sens & Act A 89 (2001) 22-27 describes a diode for use as an IR detector. Eminoglu et al. “Low-cost uncooled infrared detectors in CMOS process” describes an IR detector using diodes on a microbridge membrane fabricated in a CMOS process Sens & Act A 109 (2003) 102-113.
Similarly, bolometer-type IR detectors can be made. In U.S. Pat. No. 29,261,411 by Oulachgar et al. and U.S. Pat. No. 9,091,591 by Park et al. uncooled microbolometer detectors are disclosed.